Defibrillators are implanted in patients susceptible to cardiac arrhythmias or fibrillation. Such devices provide cardioversion or defibrillation by delivering a high voltage shock to the patient's heart, typically about 500-750V. High voltage capacitors are used in defibrillators to accumulate the high voltage charge following detection of a tachyarrhythmia. In the effort to make implantable devices as small and thin as possible, flat aluminum electrolytic capacitors are used.
Such a flat capacitor is disclosed in U.S. Pat. No. 5,131,388 to Pless et al., which is incorporated herein by reference. Flat capacitors include a plurality of aluminum layers laminarly arranged in a stack. Each layer includes an anode and a cathode, with the anodes and cathodes being commonly connected to respective connectors. The layers may be cut in nearly any shape, to fit within a similarly shaped aluminum housing designed for a particular application. With an aluminum housing, the cathode layers preferably are together connected to the housing, while the anodes are together connected to a feed-through post that tightly passes through a hole on the housing, but is electrically insulated from the housing. The feed-through post serves as an external connector for interfacing with other components.
Existing capacitors suffer a trade off in the selection of feed-through material. High purity aluminum such as 1199 alloy having 99.99% purity, is required for the anode material and for anode connecting material inside high quality aluminum electrolytic capacitors. However, this and other similarly useful alloys are inherently very soft, rendering them unsuitable for the strength required of a durable connector. A larger feed-through would have somewhat increased strength, but would sacrifice the desired miniaturization of the implantable device. Also, high purity aluminum is unsuitable for receiving solder or solder-compatible plating, as required for connection to the external portion of the feed-through. The feed-through must also be electrically conductive to avoid resistive energy loss.
The disclosed embodiment provides an aluminum electrolytic capacitor with an aluminum housing defining a chamber, and having a feed-through aperture providing communication with the chamber from outside of the housing. A number of aluminum anode layers are positioned within the chamber, and a feed-through member formed of a first conductive material occupies the feed-through aperture. The feed-through member has an inner end extending into the chamber, an outer end extending externally from the housing, and an insulative sleeve encompassing an intermediate portion of the feed-through member and directly contacting the housing at the feed-through aperture to prevent electrical contact between the feed-through member and the housing. A connection element formed of a second different conductive material is attached to the inner end of feed-through member and spaced apart from the housing. A compressible insulative gasket may be positioned between the housing and the connection element to provide insulation and a fluid seal.